Adeno-associated virus (AAV) is a small (˜26 nm) replication-defective, nonenveloped virus, that depends on the presence of a second virus, such as adenovirus or herpes virus, for its growth in cells. AAV was discovered in 1960s as a contaminant in adenovirus (a cold causing virus) preparations. Its growth in cells is dependent on the presence of adenovirus and, therefore, it was named as adeno-associated virus. AAV can infect both dividing and non-dividing cells and may incorporate its genome into that of the host cell. These features make AAV a very attractive candidate for creating viral vectors for gene therapy.